The present invention relates to an improved electrostatic copying process for repeatedly developing and transferring toner images produced from a single electrostatic image.
Electrostatic copying processes are known in the art in which an electrostatic image is formed on a photoconductive member and repeatedly developed and the resulting toner images transferred to copy sheets. These processes provide large numbers of copies of a single original document at high speed since the charging and imaging steps only have to be performed once. The spectral sensitivity of practical photoconductive materials is generally low and limits the speed at which the imaging step may be performed.
However, a major problem has existed in the prior art in that there is a large amount of charge leakage during the developing and transfer steps which result in a progressive reduction in the electrostatic image surface potential and thereby the copy density and contrast.
A prior art expedient to overcome this problem is to provide a transparent insulating layer on the photoconductive member and form the electrostatic image at the interface of the photoconductive layer and the insulating layer. However, this method requires extra process steps and it is generally difficult to form the electrostatic layer at the interface of the photoconductive layer and insulating layer rather than on the surface of a photoconductive member having no insulating layer since polarity inversion is necessary.
Another prior art process utilizes the electrostatic memorization effect of various photosensitive materials. However, these materials have low sensitivity and the imaging process is complicated and difficult.
Another prior art process utilizes a material which undergoes irreversible chemical or physical change when radiated with a light image. Although this process is desirable for making many copies of a single original document, it is excessively expensive for making only one copy since the master plate is discarded.